bit.ly is currently most popular url shortening service, so someone probably flagged it (and in general, you dont really know what was shortened, so spammers can use it too).

Good points, but in this case it’s a warning that comes from bit.ly themselves, as you can see by looking at the URL. So, I suspect that someone has reported the powerarchiver.com url as “spam or unsolicited software or web sites” to bit.ly admins, who then chose to display that warning to users.